Design Optimization and Operating Performance of S-CO₂ Brayton Cycle under Fluctuating Ambient Temperature and Diverse Power Demand Scenarios

The supercritical CO2 (S-CO2) Brayton cycle is expected to replace steam cycle in the application of solar power tower system due to the attractive potential to improve efficiency and reduce costs. Since the concentrated solar power plant with thermal energy storage is usually located in drought area and used to provide a dispatchable power output, the S-CO2 Brayton cycle has to operate under fluctuating ambient temperature and diverse power demand scenarios. In addition, the cycle design condition will directly affect the off-design performance. In this work, the combined effects of design condition, and distributions of ambient temperature and power demand on the cycle operating performance are analyzed, and the off-design performance maps are proposed for the first time. A cycle design method with feedback mechanism of operating performance under varied ambient temperature and power demand is introduced innovatively. Results show that the low design value of compressor inlet temperature is not conductive to efficient operation under low loads and sufficient output under high ambient temperatures. The average yearly efficiency is most affected by the average power demand, while the load cover factor is significantly influenced by the average ambient temperature. With multi-objective optimization, the optimal solution of designed compressor inlet temperature is close to the minimum value of 35 & DEG;C in Delingha with low ambient temperature, while reaches 44.15 & DEG;C in Daggett under the scenario of high ambient temperature, low average power demand, long duration and large value of peak load during the peak temperature period. If the cycle designed with compressor inlet temperature of 35 & DEG;C instead of 44.15 & DEG;C in Daggett under light industry power demand, the reduction of load cover factor will reach 0.027, but the average yearly efficiency can barely be improved.